This invention relates to a method and apparatus for direct casting of metal alloys from molten metal to continuous sheet or strip product. More particularly, it relates to feeding molten metal from an exit end of a casting vessel near the top of a casting roll surface to form a continuous strip of desired thickness.
In conventional production of metal strip, such methods may include the steps of casting the molten metal into an ingot or billet or slab form, then typically includes one or more stages of hot rolling and cold rolling, as well as pickling and annealing at any of various stages of the process in order to produce the desired final strip thickness and quality. The cost of producing continuous strip, particularly in as-cast gauges ranging from 0.010 inch to 0.160 inch (0.025 to 0.40 cm) could be reduced by eliminating some of the processing steps of conventional methods. The as-cast strip could be processed conventionally, by cold rolling, pickling, and annealing to various final gauges as thin as foil, for example 0.001 to 0.12 inch (0.025 to 0.30 cm).
There is a wide variety of methods and apparatus known for the production of directly cast strip. Typically such methods are those which include spraying molten metal through a metering orifice across a gap to a rapidly moving quenching surface, such as a wheel or continuous belt; methods which partially submerge a rotating quenching surface into a pool of molten metal; methods which use horizontal link belts as quenching substrates upon which molten metal flows for solidification; and methods of vertically casting with twin casting rolls having a pool of molten metal therebetween. Direct casting of metals through an orifice has long been attempted for commercial production of strip with good quality and structure, but with little success for crystalline metal strip.
More recently, other direct casting processes have been proposed but not developed into commercial processes. For example, a process is proposed for producing cold-rolled strip or sheet of austenitic stainless steel by using a continuous caster in which a casting-mold wall is moved synchronously with the cast strip and thereafter skin pass rolling as disclosed in U.S. Pat. No. 5,045,124, issued Sep. 3, 1991. Another process is disclosed in an International Application bearing No. PCT/US88/04641, filed Dec. 29, 1988 and published Aug. 10, 1989, using a melt drag metal strip casting system wherein molten metal is delivered from a casting vessel to a single chill surface such that the strip has an unsolidified top surface which is contacted by a top roll spaced a distance substantially equal to the thickness of the strip and having a temperature which will not solidify the top surface of the metal being cast. A specific tundish having flow diverters is disclosed in an International Application No. PCT/US88/04643, filed the same date and published Oct. 19, 1989. That same process-and apparatus is also disclosed in another International Application No. PCT/US90/01211, filed Mar. 14, 1990 and published Sep. 20, 1990, but further describing a grooved chill surface.
Another method is provided for directly casting molten metal from the exit end of a casting vessel onto a moving casting surface to form a continuous strip of crystalline metal using the surface tension of the molten metal for forming the top, edge, and bottom surfaces of the strip being cast with good surface quality, edges and structure. An apparatus is also provided including a casting vessel having a molten metal receiving end and an exit end from which a fully-developed uniform flow of molten metal leaves through a U-shaped structure to a moving casting surface. U.S. Pat. No. 4,678,719, issued Jul. 7, 1987, solves many problems associated with the prior art direct casting methods and apparatus such as those described above. U.S. Pat. No. 4,715,428, issued Dec. 29, 1987, describes a related method of radiantly cooling the molten metal at the exit end of the vessel.
What is still needed is a method and apparatus useful in the commercial production for direct casting strip having surface quality comparable to or better than conventionally-produced strip. Such a method and apparatus should be able to produce sheet and strip product having uniform thickness and flatness and having a smooth upper and lower surface with no porosity in the sheet. Furthermore, the method and apparatus should minimize or eliminate any handling damage of the strip after separation from the casting surface and be suitable for casting continuous strip in gauges ranging from 0.010 to 0.160 inch (0.025 to 0.40 cm). The direct cast strip should have good surface quality, edges and structure and properties at least as good as conventionally-cast strip and be suitable for the casting of carbon steels and stainless steels.